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Phosphorus Distribution from Septic Tank Effluent in Coastal Plain Soils¹

ABSTRACT

Phosphorus (P) accumulations in two soils with high perched water tables were monitored as a function of distance and depth to determine the fate of P from septic effluent in natural soil systems. The soils studied were Varina and Goldsboro sandy loams. Piezometers were placed at selected distances and depths from the drainfield in the direction of ground-water flow to determine both vertical and lateral P movement. Soil P was fractionated to determine the type and extent of soil-P fractions adjacent to the drainfield. Orthophosphate (PO₄^–3-P) concentrations in the perched ground waters decreased with distance from the septic tank drainfield in both the Varina and Goldsboro soils. PO₄^–3-P concentrations at the Varina soil location decreased from an average of 5.5 µg/ml in the undiluted effluent to 0.32 µg/ml in the soil solution at 3.0 m distance. No PO₄^–3-P was detected in waters percolating through a very slowly permeable underlying plinthic horizon. PO₄^–3-P concentrations in the Goldsboro soil solutions decreased from an average of 11.8 µg/ml in the drainline to <0.20 µg/ml at 3.0 m distance. In the Goldsboro soil P was concentrated in the 142 to 152 cm depth. Soil P fractions in the drainfield areas were present predominantly as NH₄F and NaOH extractable P with some organic-P present adjacent to the drainfield. At the Varina location, P accumulated in the argillic horizons. The NH₄F and NaOH extractable P fractions increased from 2–4 and 40–50 µ/g in the control profile to a maximum of 460 and 425 µg/g in the argillic horizons at 0.15 m, respectively. At the Goldsboro location, septic effluent had not appreciably altered the quantities of "fixed" P or the distribution of soil P fractions.

Key Words: soil P fractions • total P • organic-P • hydrolyzable P • water pollution • ground water

¹ Contribution from the Department of Agronomy, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061. This work was supported by the Virginia Department of Health.

² Assistant and Associate Professor of Agronomy, respectively.

Received for publication January 2, 1975.